This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 100 19 185.1, filed on Apr. 17, 2000, the entire disclosure of which is incorporated herein by reference.
The invention relates to a structural arrangement for aerodynamic noise reduction in connection with leading edge slats on the wings of commercial transport aircraft. Particularly, the structure provides an aerodynamic effect on a wing slat in order to reduce the noise generated by air flowing around the slat, and through the air gap between the slat and the leading edge of the wing, especially during take-off and landing phases of flight of an aircraft.
Various noise sources contribute to the total noise generated during the flight of a modern commercial transport aircraft.
Among the various noise sources, aero-acoustically generated noise that results from the flow pattern of air around the aircraft structure is becoming an evermore significant portion of the total flight noise. This is because the noise generated by other sources such as the engines has been reduced in recent years by technical advances of those components. In present day commercial transport aircraft, it is roughly estimated that approximately 50% of the total flight noise during a landing approach is generated by the flow of air around the aircraft structure, while the other half of the total noise is generated by the engines.
Further improvements, i.e. reductions, in the noise generated by the engines are only practically and economically efficacious if similar technical advances for reducing the aerodynamic flow noise around the aircraft fuselage can be simultaneously achieved. It is becoming especially important to reduce the aerodynamic flow noise in view of ever stricter noise level limits, especially around airports with a high aircraft traffic volume. A major factor contributing to the total aerodynamic flow noise during landing and take-off of a modern commercial transport aircraft, is the noise generated by the airflow around high-lift slats deployed from the leading edges of the wings during the landing and take-off phases.
To facilitate an understanding of the aerodynamic noise generated in connection with the leading edge slats, FIG. 4 of the present application shows representative streamlines of the air A flowing around a generally conventional wing, which is schematically shown in section. The wing arrangement includes a main wing 2, a leading edge slat 1 that is extended or deployed forward of the leading edge of the main wing 2, and a landing flap 11 that is extended or deployed rearward from the trailing edge of the main wing 2. Throughout this specification, the term xe2x80x9cforwardxe2x80x9d and the like refers to the normal forward flight direction of the aircraft, for example the direction in which the aircraft nose and the wing leading edges are oriented. As is generally known, the extended slat 1 and landing flap 11 change the effective camber and angle of attack of the airfoil profile of the wing structure, and also influence the airflow over the surfaces of the wing, so as to increase the lift, e.g. for landing and takeoff. In this extended configuration, the slat 1 is deployed forwardly and downwardly from the leading edge nose 2A of the main wing 2 so as to form a slat air gap 13 between a rearwardly facing concave curved surface 3 of the slat 1 and the convexly profiled leading edge nose 2A of the main wing 2.
On the other hand, during cruise flight, the slat 1 is retracted into a retracted position (not shown) directly on the leading edge nose 2A of the main wing 2 so as to reduce the aerodynamic drag and avoid unnecessary increased lift. In this context, the leading edge slat 1 must be retracted smoothly and flushly against the leading edge nose 2A of the main wing 2, so as to form a substantially continuous aerodynamic contour. Namely, the slat 1 is adjacent to the leading edge nose 2A, with at most only a small, aerodynamically insignificant, gap or space therebetween. Therefore, the rear concavely curved surface 3 of the leading edge slat 1 has a profile curvature substantially matching that of the leading edge nose 2A of the main wing 2, so that the slat 1 smoothly matches or mates onto the leading edge nose 2A of the main wing 2 without a resistance-causing gap or discontinuity therebetween.
Unfortunately, the profile curvature of the rear concave surface 3 of the slat 1 may be optimal for mating onto the leading edge nose 2A of the main wing 2 in the retracted position, but it is not optimal for the airflow through the slat air gap 13 between the leading edge nose 2A and the slat 1 in its deployed position as shown in FIG. 4. As a result, the airflow A forms an eddy or vortex 15 that extends lengthwise along the length of the slat 1 (i.e. in the wing span direction). This vortex 15 involves the turbulent eddy recirculation of air in the hollow space defined and bounded by the rear concave curvature 3 of the slat 1, whereby this space generally has a tapered concave shape or tear-drop shape. This vortex 15 further exhibits or generates a fluctuating fluid pressure field of the affected airflow, which is believed to be the cause of the aerodynamic noise generated in this area. Noise measurements in an aero-acoustic wind tunnel have confirmed that a significant reduction of the noise generated by the extended slat can be achieved by arranging a rigid fairing or filler member in the space along the rear concave curvature 3 of the slat 1, which would otherwise be occupied by the vortex 15.
Attempts have been made in the prior art to reduce the aerodynamically generated noise, especially in connection with the slats and the mounting thereof. For example, a study in this regard was published by Werner Dobrzynski and Burkhard Gehlhar entitled xe2x80x9cAirframe Noise Studies on Wings with Deployed High-Lift Devicesxe2x80x9d, from the Deutsches Zentrum fuer Luft und Raumfahrt e.V. (DLR), Institut fuer Entwurfsaerodynamik, Abteilung Technische Akustic, Forschungszentrum Braunschweig, Germany, at the Fourth American Institute of Aeronautics and Astronautics AIAA/CEAS Aeroacoustics Conference on Jun. 2 to 4, 1998 in Toulouse, France.
Among other things, this study disclosed a proposed noise reducing arrangement in which a sheet metal guide member is pivotally connected to the slat in the area of the concavely curved rear or inner surface of the slat facing toward the leading edge nose of the main wing. This sheet metal air guide can be pivoted relative to the slat. Particularly, the air guide member can be extended or deployed relative to the slat during take-off and landing when the slat is deployed relative to the wing. On the other hand, the sheet metal air guide member will be pivoted against the slat during cruise flight when the slat is to be retracted relative to the wing. While such a proposed solution may have achieved a reduction of aerodynamically generated noise in wind tunnel tests, it is considered that such a solution could never be practically carried out in an actual aircraft construction, for practical reasons.
For example, in the previously proposed arrangement, when the slat is retracted against the leading edge nose of the main wing for cruise flight, the gap between these two components is not sufficiently large for accommodating a rigid air guide member tilted or pivoted inwardly against the rear surface of the slat. On the other hand, if the gap is made larger to accommodate the air guide member, then a disadvantageous aerodynamic gap or discontinuity would be formed along the aerodynamic contour provided by the slat and the wing in combination. Moreover, if a flexible air guide component is provided, which is to be adapted against the inner contour of the slat in the retracted position, then such a component would not have sufficient strength and stiffness in order to withstand the aerodynamic forces in the deployed condition.
Moreover, such a guide element would be expected to have a tendency to flutter due to the alternating aerodynamic pressure effect, or simply due to a failure to remain sufficiently rigid to withstand the aerodynamic forces. Namely, the proposed sheet metal separating surface or air guide member will be subjected to considerable fluctuating aerodynamic forces, which will presumably excite vibrations or oscillations in the member, since it is only to be pivotally connected to the lower edge of the slat without any further stiffening means. Such fluttering generates a significant noise radiation, which is directly contrary to the object of reducing the noise. Furthermore, a pivotally connected sheet metal member requires additional mechanical movable parts, which leads to an increased total weight of the aircraft, as well as increased manufacturing and maintenance costs. It would also be necessary to construct the pivot joint in such a manner that the transition from the underside of the slat to the joint of the separating surface is free of contour discontinuities or gaps, which makes it necessary to achieve a very high manufacturing accuracy.
Additional problems arise because the contour of the rear surface of the slat as well as the geometry of the slat air gap change over the span width of the wing, so that the air guide element or elements must be configured with a bend or twist along the length thereof, whereby the tilting and retracting mechanism becomes further complicated.
A failure situation, for example involving a blockage of the mechanical system of the slat arrangement, would become very critical, because then the slat could no longer be retracted if the air guide member is blocked or jammed in its deployed or extended position.
The above mentioned conference proceedings provide no suggestions toward overcoming the just mentioned significant problems and disadvantages in actually trying to carry out the proposed solution using a pivotable air guide member in practice.
In view of the above, it is an object of the invention to provide a structure for reducing the aero-acoustically generated noise of the air flowing around a slat, which significantly reduces the noise generation on the slat of a commercial transport aircraft, without disadvantageously influencing the aerodynamic characteristics such as the lift and the air resistance of the wing.
Moreover, the structure or arrangement should be fail-safe, so that in the event of a total or partial failure of the arrangement, no dangerous effects that would influence the further flight of the aircraft may arise. Furthermore, the inventive structural arrangement shall entirely avoid the use of movable mechanical elements and elements that significantly increase the total weight of the aircraft. The inventive structural arrangement shall also be easily installable or even retrofittable into existing aircraft, and have a simple and minimal maintenance requirement. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in an aircraft wing arrangement including a main wing and a slat that is movably connected to the main wing so as to be movable between an extended position in which the slat extends forwardly of the leading edge nose of the wing and a retracted position in which the slat is arranged adjacent to the leading edge nose. Particularly according to the invention, the aircraft wing arrangement includes an improved structure for reducing aero-acoustically generated noise, comprising a displacement element (which is especially and preferably an expandable displacement element such as a hollow bellows or inflatable elastic displacement element) that is arranged on the concavely curved rear surface of the slat that faces toward the leading edge nose of the main wing. The inventive improvement further comprises a pressure conduit such as a bleed air line that communicates with a hollow space in the displacement element so as to selectively expand or contract the displacement element. The terms xe2x80x9cconduitxe2x80x9d, xe2x80x9clinexe2x80x9d, etc. refer generally to any duct, hose, pipe, tube, channel, conduit or the like through which a fluid may be conveyed. Any available pressurized fluid and preferably pressurized air can be used to selectively expand the displacement element. Preferably, compressed bleed air from the engine or engines of the aircraft is controlledly supplied into the expandable displacement element.
When the displacement element is completely contracted, it is rather thin and contracted against the concave rear surface of the slat, so that it can be accommodated in the crescent-shaped or sickle-shaped space (referring to the cross-sectional shape) between the slat and the leading edge nose of the main wing when the slat is fully retracted. On the other hand, when the displacement element is fully expanded, it preferably has a substantially teardrop shaped cross-section, which protrudes rearwardly from the concavely curved rear surface of the slat and forms a protruding convexly curved surface of the displacement element that faces the leading edge nose of the main wing. Thus, when the slat is in the extended position, the slat air gap between the slat and the leading edge nose of the main wing is bounded between the convexly curved rear surface of the expandable displacement element and the aerodynamically convexly curved profile contour of the leading edge nose. The rear surface of the expandable displacement element may further have a compound curvature including convex and concave portions, to optimize the flow configuration of the slat air gap.
The inventive improved structure achieves several advantages. The expandable displacement element improves or even optimizes the flow configuration of the slat air gap, so that the aerodynamic effectiveness of the slat can be improved, and especially so that the formation of a vortex on the rear surface of the slat is avoided, thereby significantly reducing the aero-acoustic generation of noise from the slat. On the other hand, when the expandable displacement element is deflated and thus contracted, it is rather thin and conformable to the concavely curved contour of the rear surface of the slat, so that it can be compactly accommodated between the slat and the leading edge nose of the wing when the slat is in the fully retracted position. In this configuration, the displacement element flexibly deformably adapts to the available space between the retracted slat and the leading edge nose of the wing and thus advantageously fills any gap remaining between the slat and the wing. The structure is economical and simple to fabricate and to install, and may even be easily retrofitted onto existing aircraft. The added weight is quite low. The arrangement provides a fail-safe operation, whereby even a complete failure of the arrangement will not hinder the normal retraction or extension of the slat relative to the wing.